Numerous types of recuperative heat exchangers for air flows are known. They usually have stationary plates which are positioned in parallel and are made, for example, of glass or metal and they separate from one another the two air flows between which the heat exchange is to take place. Heat transfer occurs because of the good heat conductivity of the plates, which may also have a profiled surface for this purpose, if they are made of metal. In order to further enlarge the heat exchange surface, it is also known to position several plates parallel to each other with the two air flows being guided alternately through the slits between the plates. This gives a compact design which is economically valuable.
The known heat exchangers mentioned above may be installed, for example, in channels with a square or rectangular cross section, but are less suitable for installation into narrow air slits, because their heat-exchanging surface is too small for this purpose. Another disadvantage of these known heat exchangers is that heat conduction not only takes place perpendicularly to the plates separating the air flows, but also takes place in the longitudinal direction of the plates due to the good heat conductivity thereof. However, this phenomenon is undesirable because it reduces the temperature difference by virtue of which the air flows are cooled or heated when flowing through the heat exchanger. The greater this temperature difference, the higher the efficiency of the heat exchanger A further disadvantage is that the heat from the air flows is only transferred from their layers bordering on the plates and accordingly, the cross section of flow should be as small as possible. Consequently, the heat exchanger should be fitted with as many heat exchange plates as possible, which correspondingly increases the cost. If the surface is profiled to a substantial extent, a turbulent air flow is produced, as a result of which, however, the flow resistance is considerably increased, so that the flow velocity is reduced with a consequent reduction in the efficiency of the heat exchanger.
U.S. Pat. No. 3,895,675 discloses a breathing apparatus which, in a hollow housing made of poorly heat-conducting material, such as glass fibre reinforced polyester, contains dividing walls extending in longitudinal direction made of the same material and contains pins penetrating these dividing walls perpendicularly to the direction of flow, the pins being made of good heat-conducting material, such as copper and extending from wall to wall through the channel and the dividing walls and ensuring heat transfer between the neighbouring flow sections of the heat exchanger which is designed for intermittent operation. In this manner, storage of heat in, and heat conduction along, the dividing walls is reduced, and it ensures that heat exchange does not only take place between the regions of the air flows directly bordering on the dividing walls. However, because the dividing walls of the heat exchanger are relatively rigid, the heat exchange pins have to be designed so that they fit exactly and have to be installed in corresponding openings. A heat exchange takes place at all events, but moisture is not also transferred between the air flows which are in counterflow to one another.